[DOCS] Adds semantic search endpoint to the list of trained model APIs (elastic#2270)

Author: szabosteve

docs/en/stack/ml/nlp/ml-nlp-apis.asciidoc

@@ -27,3 +27,14 @@ All the trained models endpoints have the following base:
 // UPDATE
 * {ref}/put-trained-models-aliases.html[Update trained model aliases]
 
+
+// SEMANTIC SEARCH
+The Semantic search endpoint has the following base:
+
+[source,js]
+----
+<target>/_semantic_search
+----
+// NOTCONSOLE
+
+* {ref}/semantic-search-api.html[Semantic search]